1. Technical Field
The present invention relates to an image forming apparatus and an image forming method that form an image by developing a latent image formed on a photoreceptor with a liquid developer, such as toner or carriers, and transferring the developed image onto a recording material.
2. Related Art
Various wet type image forming apparatuses that develop a latent image, using a high-viscosity liquid developer formed by dispersing toner made of solid components into a liquid solvent, and visualize an electrostatic latent image have been proposed. The developer used in the wet type image forming apparatuses is made by suspending solid content (toner particles) into a high-viscosity organic solvent (carrier liquid) having electric insulation, which is made of silicon oil, mineral oil, or food oil, in which the diameter of the toner particles is very small, around 1 μm. It is possible in the wet type image forming apparatuses to achieve high quality in comparison to dry type image forming apparatuses using a powder type of toner particles with particle diameter of about 7 μm, by using fine toner particles.
In the image forming apparatuses using the liquid developer, it has been attempted to efficiently use the liquid developer in various ways by reusing the liquid developer that has not contributed to visualizing the electrostatic latent image.
An image forming apparatus that circulates a liquid developer by supplying the liquid developer to a supply unit in a developer container from a developer collecting-replenishing unit, collecting the liquid developer overflowing the supply unit through a partition into a collecting unit, and returning the collected liquid developer to the developer collecting-replenishing unit is disclosed in JP-A-2009-75552.
An image forming apparatus equipped with an ultrasonic vibrator provided with at least one of a developing tank, a liquid toner tank, a liquid toner supply unit, and a liquid toner collecting unit is disclosed in JP-A-2000-330385. It is possible to prevent waste of toner and deterioration of the quality of a developed image due to toner aggregation substances by dispersing even toner particles, which can be reused in developing, even if toner aggregation substances are produced in a liquid developer, by applying ultrasonic vibration to the liquid developer from the ultrasonic vibrator.
It is disclosed in JP-A-2000-330385 that it is possible to prevent aggregation of liquid toner by disposing an ultrasonic vibrator in a circulation system of liquid toner, but the ultrasonic vibrator is just uniformly driven in the liquid developing device. The state of the liquid toner (liquid developer) is changed by various process conditions in image forming and it is difficult to cope with the state of the liquid developer when the ultrasonic vibrator is uniformly driven.
Further, when ultrasonic vibration is applied to the liquid developer, the liquid developer increases in temperature and correspondingly changes in viscosity. When excessive ultrasonic vibration is applied in consideration of the worst aggregation of the liquid developer, the temperature excessively increases, viscosity correspondingly changes, and the formed image is deteriorated.